Stable microdispersions and microgels based on acrylic polymers, method for obtaining them and compositions, particularly cosmetic compositions, containing them

ABSTRACT

The invention relates to a process for the preparation of a stable microdispersion of particles comprised of acrylic polymers in an organic solvent used as reaction medium in the presence of a stabilizing agent comprised of a block copolymer, characterized in that said acrylic polymer is obtained by radical polymerization of at least one acrylic monomer in the presence of a block copolymer based on polymethyl methacrylate (PMMA) and polytert-butyl acrylate (PtBuA). It also relates to microdispersions which may be obtained by said process as well as to microgels produced from said microdispersions and compositions, particularly cosmetic compositions such as nail varnish.

The present invention relates to novel stable microdispersions ofpolymer particles, to the microgels obtained from these dispersions andto compositions, particularly cosmetic compositions, containing thesemicrodispersions or microgels. It further relates to a method ofpreparing these dispersions and microgels.

In terms of the invention, microdispersion is understood as meaning adispersion of particles whose size can range up to 120 nm and ispreferably less than 100 nm. The polymer constituting these particles isin precipitated form in the reaction medium, the particles beingstabilized by a copolymer.

In line with Antonietti et al. in "Makromol. Chem., Makromol. Symp." 30,81-93 (1989), a microgel is defined according to the invention as beinga micro-particle of crosslinked polymer, in swollen form, dispersed inan organic solvent; its size, which is governed by the swelling ratio,depends on the crosslinking density. The polymer of which the microgelis composed, in the non-crosslinked form, is soluble in this samesolvent; only crosslinking prevents the particles from being dissolved.

The essential difference between a microdispersion and a microgel is thefact that the former exists in precipitated form in the reaction mediumformed by a solvent which does not solubilize the polymer, whereas themicrogel is in swollen form in solvents for the polymer.

By way of example, a PMMA microdispersion prepared in an alcoholicreaction medium exists in precipitated form because the alcohol is not asolvent for PMMA, the microdispersion in precipitated form beingstabilized by the block copolymer. Being crosslinked, these sameparticles make it possible to obtain microgels in swollen form when theyare transferred into a solvent for PMMA, such as butyl acetate.

Methods of preparing acrylic microgels in aliphatic hydrocarbons, eitheron their own or mixed with other hydrocarbons or with alcohols, havebeen described in a number of publications.

Thus patent U.S. Pat. No. 4 558 075 describes the preparation of acrylicmicroparticles in the presence of PGMA-PMMA-PMAA random copolymers in areaction medium consisting of heptane, and their application in thefield of automotive paint containing aluminum flakes. The polymerparticles have dimensions of between 0.01 and 10μ.

D. H. EVERETT and J. F. STAGEMAN (Discus. Faraday Soc. 65, 230, 1978)have described the preparation of polystyrene or PMMA dispersions in thepresence of PDMS-b-PS-b-PDMS three-block copolymers using alkanes as thereaction medium. The polymer particles obtained according to saiddocument have dimensions of the order of 0.1μ.

J. V. DAWKINS, S. A. SHAKIR and T. G. CROUCHER, Europ. Polym. J. 23,173-175, 1987, have described the synthesis of PMMA dispersions incyclohexane in the presence of PS-PMMA copolymers as stabilizers.

German patent DE 3 439 128 describes the preparation of acrylicmicrogels, stabilized either by polyesters or by copolymers of the"polyhydroxystearic acid-PMMA-PAA" type, in a reaction medium consistingof water or aliphatic hydrocarbons, and their applications in thepreparation of metallic lacquers based on aqueous microgels or whitelacquers based on titanium oxide in the case of microgels in aliphatichydrocarbons. The dimensions of the polymer particles described in saiddocument are of the order of 0.01 to 10μ.

Other publications cite the use of mixtures of hydrocarbons and alcoholsas the reaction medium. This is the case of European patent EP 251 553,in which acrylic microgels are prepared in the presence ofcopolymerizable stabilizers, which are copolymerized with acrylicmonomers to produce a core-shell. The dispersions obtained can be usedfor painting metals.

This is also the case of the publication by C. H. BAMFORD et al., J.Appl. Polym. Sci., 25, 2559-2566, 1980, which again describes microgelsin a solvent medium consisting of a mixture of alcohol and hydrocarbon,except that the use of a surface-active stabilizer is dispensed with.

It therefore appears that, as far as acrylic microgels are concerned,the stabilizing copolymers are generally graft or random copolymersbased on polymethyl methacrylate. The publication by J. V. DAWKINS etal., Euro. Polym. J. 23, 173-175, 1987, uses PMMA-b-PS block copolymersas stabilizers for polymethyl methacrylate dispersions.

British patent GB 941 305 describes the preparation of acrylic polymers,stabilized by block copolymers which are also acrylic, in ethyl alcohol.The stabilizing part of the copolymer employed in said patent iscomposed of polymethacrylic acid. The sizes of the particlesconstituting the dispersions described in patent GB 941 305 are of theorder of 0.2μ or more. Furthermore, the dispersions obtained byfollowing the teaching of said patent have dry extracts of between 25and 65%.

In the course of its researches, the Applicant has discovered that, byusing as the stabilizer a block copolymer based on polymethylmethacrylate (PMMA) and polytert-butyl acrylate (PtBuA), it is possibleto prepare, in an alcoholic medium, microdispersions of acrylic polymerparticles whose dimensions are much smaller than those describedhitherto, being less than 120 nm and generally of the order of 80 nm,which is of great advantage in terms of the stability of saiddispersion.

These particles also have a low size polydispersity index characterizedby the ratio Dw/Dn, where Dw is the weight-average size and Dn thenumber-average size.

By virtue of their small size, these particles have a large specificsurface area, which can be a favorable factor in compositions containingpigments or fillers.

Furthermore, the microdispersions of the invention also have theadvantage of being able to be concentrated, or even dried, andredispersed in a solvent, for example an alcohol, while retaining thesame characteristics.

Another advantage of the microdispersions according to the invention isthat they are compatible with esters to form microgels.

The advantages and characteristics of the invention are apparent fromthe following description.

According to one of these essential characteristics, the inventionrelates to a method of preparing a stable microdispersion of particlesconsisting of acrylic polymers, in an organic solvent used as thereaction medium, in the presence of a stabilizer consisting of a blockcopolymer, characterized in that said acrylic polymer is obtained by thefree-radical polymerization of at least one acrylic monomer in thepresence of a block copolymer based on polymethyl methacrylate (PMMA)and polytert-butyl acrylate (PtBuA).

Thus the method of the invention consists in preparing an acrylicpolymer by a known method of free-radical polymerization in a solvent,and in carrying out this polymerization in the presence of a blockcopolymer made up of PMMA and PtBuA.

This block copolymer advantageously has a molecular weight of between20,000 and 500,000, preferably of between 40,000 and 200,000.

It can be either a two-block copolymer symbolically represented byPMMA-b-PtBuA, or a copolymer of the three-block type symbolized byPtBuA-b-PMMA-b-PtBuA.

In the case of the double block, the percentage by weight of thepolytert-butyl acrylate block will be between 10 and 90% but preferablyclose to 50%.

In one advantageous variant of the method, the acrylic monomer isselected from the family of alkyl acrylates and alkyl methacrylateshaving linear or branched C₁ to C₁₈ alkyl groups, or mixtures thereof.Methyl, butyl and lauryl groups may be mentioned as examples of alkylgroups. Methyl methacrylate, butyl acrylate or a mixture of these twomonomers may be mentioned as preferred examples of acrylic monomers.Mixtures of methyl methacrylate and lauryl methacrylate may also bementioned.

These acrylic monomers will advantageously be copolymerized, accordingto the method of the invention, with a difunctional monomer.

Divinylbenzene and diacrylic or dimethacrylic monomers, for examplebutanediol dimethacrylate, ethylene glycol dimethacrylate, diethyleneglycol dimethacrylate and tetraethylene glycol dimethacrylate, may bementioned as examples of difunctional monomers.

A preferred difunctional monomer according to the invention isbutanediol dimethacrylate (BDMA).

The amount of block copolymer used to form the microdispersions of thepresent invention is advantageously between 1 and 50% by weight,preferably between 5 and 30%, and more preferably of the order of 10%based on all the monomers used to prepare said acrylic polymer.

The reaction medium used is an alcohol, in particular a C₁ C₄ alcohol,preferably ethanol, or a mixture of alcohols defined above, for examplea mixture of ethanol and isopropanol.

The reaction medium used can also be any solvent which solubilizes thePtBuA chain of the stabilizing copolymer and which precipitates the PMMAchain of this same copolymer and the acrylic polymer formed. Analcohol-rich medium can be used in particular, examples being a mixtureof ethanol with 20% by volume of butyl acetate or a mixture of ethanolwith 20% by volume of ethyl acetate.

The free-radical polymerization is carried out in conventional manner inthe presence of a free-radical polymerization initiator consisting of anorganosoluble initiator preferably selected from the family of azoinitiators and peroxides.

Azobisisobutyronitrile and benzoyl peroxide may be mentioned as examplesof preferred initiators.

Advantageously the free-radical polymerization is carried out in severalsteps comprising the preparation of a seed followed by the gradualaddition of the remaining products.

The seed will preferably consist of about 20% of the mixture of monomersand crosslinking agent and about 50% of the initiator. This mixture willbe added to the preformed micellar solution of the copolymer in alcoholat the polymerization temperature; the polymerization is preferablycarried out at about 5° to 10° C. below the boiling point of the solventconstituting the reaction medium, and is allowed to proceed for about 4h.

The remaining constituents, but preferably all the initiator remainingdissolved in alcohol, and the monomers will be added slowly to theresulting seed over a period advantageously of between 2 and 4 h.

The synthesis will be stopped after an additional polymerization time ofthe order of 16 h.

The above-described microdispersions of acrylic polymers have particledimensions and a monodispersity which have never been attained hithertofor dispersions in an alcoholic medium. They constitute industrialproducts which are novel per se.

Thus, according to another of its aspects, the invention relates to theacrylic polymer dispersions which can be obtained by the methoddescribed above.

The dispersions are composed of particles having dimensions of less than120 nm, generally of the order of 80 nm, and a low polydispersity index(between 1.1 and 1.2).

Furthermore, these microdispersions have the following advantages:

the microdispersions are compatible with esters, such as butyl acetate,to form microgels;

the products obtained are compatible with nitrocellulose dissolved inbutyl acetate to give a transparent and glossy film;

the polymer obtained does not contain acid groups, these generally beingdetrimental to nitro-cellulose-based formulations;

crosslinking of the particles enables the rheological characteristics tobe adjusted; and

the microdispersions prepared in this way can be used to obtain nailvarnishes in an alcoholic medium.

Thus, according to another of its aspects, the invention relates to themicrogels obtained from the microdispersions of the invention. Moreprecisely, these microgels are obtained by transferring the particlesinto a solvent for PMMA, either after prior drying of themicrodispersions or by adding this same solvent to the microdispersionconcentrated beforehand.

Solvents which may be mentioned are aromatic solvents, chlorinatedsolvents such as chloroform or methylene chloride, ketones and esterssuch as C₂ to C₄ alkyl acetates, more particularly butyl acetate andethyl acetate.

Principally to avoid obtaining excessively viscous microgels, and henceto facilitate their use, the concentration by weight of polymers in themicrogels, in particular when the latter are intended for theformulation of varnishes such as nail varnishes, does not generallyexceed about 30%. This proportion is preferably between about 10% and20%.

The dispersions and microgels of the invention prove particularly usefulin the fields of paint and cosmetics, where they enable the rheologicalcharacteristics to be adjusted. In fact, the microdispersion can bemixed with nitrocellulose-based formulations of high dry extract withoutappreciably increasing the viscosity of the resulting system.

The fact that the products are compatible with nitrocellulose dissolvedin butyI acetate to give a transparent and glossy film makes it possibleto contemplate their use for the preparation of varnishes, particularlynail varnishes.

More precisely, the use of the microgels of the invention in varnishformulations, especially nail varnish formulations, has the followingadvantages in particular:

it enables the rheological properties to be improved, making it possiblein particular to avoid precipitation of the pigments, and enables thereproducibility of these properties to be improved. In particular, thismakes it possible substantially to reduce the amount of, or evendispense with, the bentone which is generally used for this purposedespite its well-known disadvantages;

it enables the dry extract of the film formed by the varnish to beincreased without thereby substantially increasing the viscosity of thevarnish;

it gives the film more gloss; and

it reinforces the thixotropic effect provided by bentone in an acetatemedium.

The proportion by weight of microgel according to the invention in thefinal composition of the varnish can range up to about 30%, for examplein the case where it is desired to reduce the amount of nitrocelluloseor dispense with it. In general, however, it is preferred to useproportions of between about 5 and 20% by weight.

The Examples which follow are given purely in order to illustrate theinvention.

EXAMPLES Example 1 Preparation of a Microdispersion According to theInvention

The two-block copolymer used in this Example (copolymer I) has anumber-average molecular weight of 90,000 and comprises 50% by weight ofPtBuA; the PtBuA and PMMA blocks have Mn values of 45,000 each.

In a jacketed reactor equipped with a condenser and a nitrogen inlet,150 g of ethanol and 0.76 g of copolymer are heated at 65° C. for 30minutes, with stirring.

0.2 g of azobisisobutyronitrile (AIBN), 1.5 g of methyl methacrylate(MMA) and 0.075 g of butanediol dimethacrylate (BDMA) are then added andpolymerization is allowed to proceed for 4 h.

0.2 g of AIBN dissolved in 20 ml of ethanol is introduced, this beingfollowed by the addition of 5.70 g of MMA and 0.29 g of BDMA at a rateof 2.5 ml/h. When the addition is complete, the polymerization iscontinued for about 16 h. The microdispersion obtained has a dry extractof 4.1%, the particles have a size of 98 nm and the value of Dw/Dn is1.12.

Example 2 Preparation of a Microdispersion

The method of synthesis is the same as that given for Example 1, theonly change being the proportions of the various constituents.

Preparation of the seed:

    ______________________________________                                        150         g           EtOH                                                  1.87        g           copolymer I                                           0.2         g           AIBN                                                  0.53        g           BuA                                                   0.97        g           MMA                                                   0.075       g           BDMA                                                  ______________________________________                                    

followed by the addition of 0.2 g of AIBN in 20 ml of EtOH and then bythe addition of:

    ______________________________________                                        2.0         g             BuA                                                 3.7         g             MMA                                                 0.29        g             BDMA                                                ______________________________________                                    

at 2.5 ml/h.

The dispersion obtained has a dry extract of 4.7%, the particlesobtained have a size of 86.5 nm and the value of Dw/Dn is 1.14.

Example 3 Demonstrating the Possibility of Using a Copolymer of HigherMolecular Weight

The two-block copolymer employed in this Example (copolymer II) has anumber-average molecular weight of 150,000 and comprises 75% by weightof PtBuA; the PtBuA and PMMA blocks have Mn values of 112,500 and 37,500respectively.

The procedure is unmodified, the proportions of the various constituentsbeing as follows:

Preparation of the seed:

    ______________________________________                                        150         g           EtOH                                                  1.87        g           copolymer II                                          0.2         g           AIBN                                                  0.75        g           MMA                                                   0.75        g           BuA                                                   0.075       g           BDMA                                                  ______________________________________                                    

followed by the addition of 0.2 g of AIBN in 20 ml of EtOH and then bythe addition of:

    ______________________________________                                        2.85 g               MMA                                                      2.85 g               BuA                                                      0.29 g               BDMA                                                     ______________________________________                                    

at 2.5 ml/h.

The dispersion obtained has a final dry extract of 4.7%, a particle sizeof 94.3 nm and a Dw/Dn value of 1.15.

Example 4 Demonstrating the Possibility of Using a PMMA-Rich Copolymer

The two-block copolymer used here (copolymer III) has a number-averagemolecular weight of 80,000 and comprises 35,4% by weight of PtBuA; thePtBuA and PMMA blocks have Mn values of 28,300 and 51,700 respectively.

The course of the synthesis remains unchanged, the ingredientsintroduced being as follows:

Preparation of the seed:

    ______________________________________                                        150        g            EtOH                                                  0.76       g            copolymer III                                         0.2        g            AIBN                                                  0.75       g            BuA                                                   0.75       g            MMA                                                   0.075      g            BDMA                                                  ______________________________________                                    

followed by the addition of 0.2 g of AIBN in 20 ml of EtOH and then bythe addition of:

    ______________________________________                                        2.85 g               MMA                                                      2.85 g               BuA                                                      0.29 g               BDMA                                                     ______________________________________                                    

at 2.5 ml/h.

The dispersion obtained has a solids content of 4.3%, a particle size of81.3 nm and a Dw/Dn of 1.14.

Example 5 Demonstrating the Influence of the Average Molecular Weight ofthe Block Copolymer

The two-block copolymer IV used for this Example has a total Mn of26,000, the percentage by weight of the PtBuA block being 85%.

The method of synthesis is the same, the proportions involved being asfollows:

Preparation of the seed:

    ______________________________________                                        250        g            EtOH                                                  3          g            copolymer IV                                          1.40       g            MMA                                                   0.07       g            BDMA                                                  0.1        g            AIBN                                                  ______________________________________                                    

followed by the addition of 0.4 g of AIBN in 20 ml of EtOH and then bythe addition of:

    ______________________________________                                        13.34 g              MMA                                                       0.29 g              BDMA                                                     ______________________________________                                    

at 2.5 ml/h.

However, the dispersion has a substantially lower stability than thedispersions obtained with the polymers of Mn≧40,000.

Example 6 Preparation of a Microdispersion from an Acrylic MonomerMixture

The two-block copolymer used in this Example has a number-averagemolecular weight of 53,000 and comprises 85% by weight of PtBuA and 15%of PMMA.

In a jacketed reactor equipped with a condenser and a nitrogen inlet,150 g of an isopropanol/ethanol mixture containing 80% of isopropanoland 20% of ethanol, and 0.76 g of two-block copolymer, are heated at 65°C. for 30 minutes, with stirring.

0.2 g of AIBN, 1.62 g of MMA, 0.18 g of lauryl methacrylate (LMA) and0.075 g of BDMA are then added. The polymerization is allowed to proceedfor 4 h.

0.2 g of AIBN dissolved in 20 ml of an 80/20 isopropanol/ethanol mixtureis introduced and 6.48 g of MMA, 0.72 g of LMA and 0.29 g of BDMA areadded at a rate of 2.5 ml/h. When the addition is complete, thepolymerization is continued for about 16 h. The microdispersion obtainedhas a dry extract of about 5.5% and the constituent particles have asize of about 108 nm.

Example 7 Demonstrating the Possibility of Using a Three-Block Copolymerof the PtBuA-b-PMMA-b-PtBuA Type

The three-block copolymer, copolymer V, has a number-average molecularweight of 30,000 and a percentage of PtBuA of 54% and the PMMA has an Mnof 14,000.

The course of the synthesis is the same as previously, the amountsintroduced being as follows:

Preparation of the seed:

    ______________________________________                                        150         g             EtOH                                                0.75        g             BuA                                                 0.075       g             BDMA                                                ______________________________________                                    

followed by the addition of 0.2 g of AIBN in 20 ml of EtOH and then bythe addition of:

    ______________________________________                                        2.85 g               MMA                                                      2.85 g               BuA                                                      0.29 g               BDMA                                                     ______________________________________                                    

at 2.5 ml/h.

The dispersion obtained has a dry extract of 4%, a particle size of 101nm and a Dw/Dn of 1.13.

Example 8 Preparation of the Microgels and Compatibility withNitrocellulose

There are several possible modes of preparing microgels. The simplestconsists in drying the microdispersion completely on a rotary evaporatorand redispersing the polymer in the appropriate amount of solvent forPMMA, for example butyl acetate, to give the microgel at the desiredconcentration.

Another method of obtaining the microgels consists in concentrating themicrodispersion on a rotary evaporator and directly adding the amount ofsolvent for PMMA, for example butyl acetate, which is necessary tosolubilize the PMMA. It is generally considered that the amount ofsolvent for PMMA has to represent a volume of more than 20% of the totalvolume of solvent.

In this same method, a solution of nitrocellulose dissolved in butylacetate can also be added directly to the concentrated microdispersion.

By way of example, the latter method can be illustrated as follows:

The microdispersion obtained in Example 3 is concentrated on a rotaryevaporator at 40° C. until a microdispersion with a concentration of 20%in ethanol is obtained. This has a viscosity of 7.0.10⁻³ Pa.s (7 cP) at20° C., compared with 1.2.10⁻³ Pa.s (1.2 cP) for ethanol alone at thesame temperature.

A mixture of equal proportions of the same 20% microdispersion inethanol and a 20% solution of nitrocellulose of the type CA4 A20 inbutyl acetate, with a viscosity of 17.9.10⁻³ Pa.s (17.9 cP), has aviscosity of 31.6.10⁻³ Pa.s (31.6 cP).

The solutions obtained are limpid and do not exhibit any phaseseparation, even on storage, demonstrating the compatibility between themicrogels and nitrocellulose.

Example 9 Preparation of Nail Varnishes

Different nail varnishes are prepared by introducing into anitrocellulose base a microgel according to the invention obtained fromthe microdispersion of Example 1 by dispersing the polymer in butylacetate using a stirrer of the deflocculating type. The proportion ofmicrogel is about 5 to 20%, based on the total weight of the varnishprepared in this way.

By way of example, the nitrocellulose bases used to prepare the nailvarnishes have the following compositions in percentages by weight:

    ______________________________________                                        nitrocellulose            10 to 20                                            arylsulfonamide resin (Santolite ® or                                                               10 to 15                                            Lustralite ®)                                                             polyester resin           2 to 5                                              solvents (butyl acetate/ethyl acetate/                                                                  60 to 80                                            toluene mixture comprising less than 40%                                      of toluene)                                                                   plasticizer:              0.5 to 8                                            dibutyl phthalate                                                             Citroflex-A2 ®                                                            camphor                                                                       suspending agent, for example bentone                                                                   0 to 1.5                                            pigments                  0 to 2                                              ______________________________________                                    

Example 10 Nail Varnish

A nail varnish formulation comprising the following proportions inpercentages by weight is prepared according to Example 9:

    ______________________________________                                        nitrocellulose        15                                                      microgel according to the invention                                                                 15                                                      dibutyl phthalate     1                                                       solvent (composed of 55% of ethyl                                                                   68                                                      acetate, 15% of butyl acetate and                                             30% of toluene)                                                               pigments              1                                                       ______________________________________                                    

What is claimed is:
 1. In a method of preparing a stable microdispersionof particles of acrylic polymers in an organic solvent in the presenceof a stabilizer the improvement comprising: forming said acrylic polymerby a free-radical polymerization of at least one acrylic monomer in thepresence of a block copolymer as said stabilizer, wherein said blockcopolymer is a copolymer of polymethyl methacrylate (PMMA) andpolytert-butyl acrylate (PtBuA) and said solvent solubilizes said PtBuAchain of said copolymer and precipitates both said PMMA chain of saidcopolymer and said acrylic polymer.
 2. The method according to claim 1wherein said block copolymer has a molecular weight between 20,000 and500,000.
 3. The method according to claim 1 wherein the block copolymeris a two-block copolymer and PtBuA represents 10 to 90% by weight ofsaid block copolymer.
 4. The method according to claim 1 wherein saidblock copolymer is a three-block copolymer of PtBuA-b-PMMA-b-PtBuA. 5.The method according to claim 1 wherein said acrylic monomer is an alkylacrylate or an alkyl methacrylate, said alkyls being linear or branchedC₁ to C₁₈ alkyls, or a mixture of said monomers.
 6. The method accordingto claim 1 wherein the acrylic polymer is a copolymer of at least oneacrylic monomer and a difunctional monomer.
 7. The method according toclaim 6 wherein the difunctional monomer is butanediol dimethacrylate(BDMA).
 8. The method according to claim 1 wherein the block copolymeris present in an amount between 1 and 50% by weight based on themonomer.
 9. The method according to claim 1 wherein the solvent is analcohol, a mixture of alcohols, or an alcohol-rich medium.
 10. Themethod according to claim 9 wherein said solvent is ethanol.
 11. Themethod according to claim 1 wherein said free-radical polymerization iscarried out in the presence of an initiator belonging to the family ofazo compounds and peroxides.
 12. The method according to claim 11wherein said initiator is azobisisobutyronitrile or benzoyl peroxide.13. The method according to claim 1 wherein said polymerizationcomprises sequential steps of preparing a seed followed by graduallyadding the remaining reactants.
 14. A microdispersion of acrylicparticles obtained by the method according to claim
 1. 15. A method formaking a microgel comprising the steps of: drying or concentrating amicrodispersion obtained by the method of claim 1; and then combiningthe dried or concentrated microdispersion with a solvent for PMMA toform a microgel.
 16. A microgel made by a process comprising the stepsof:(a) forming a stable microdispersion of particles of acrylic polymersin an organic solvent wherein said acrylic polymer is formed by afree-radical polymerization of at least one acrylic monomer in thepresence of a block copolymer, said block copolymer being a copolymer ofpolymethyl methacrylate (PMMA) and polytert-butyl acrylate (PtBuA); (b)drying or concentrating said stable microdispersion of particles, andthen transferring said dried or concentrated stable microdispersion ofparticles to a solvent for PMMA to form a microgel; and (c) recoveringsaid microgel.
 17. The microgels according to claim 16 wherein saidsolvent for PMMA is selected from the group comprising aromaticsolvents, chlorinated solvents such as chloroform or methylene chloride,ketones and esters such as a C₂ to C₄ alkyl acetate.
 18. The microgelsaccording to claim 16 wherein said solvent for PMMA is butyl acetate orethyl acetate.
 19. The microgels according to claim 16 wherein theconcentration by weight of polymer in the solvent for PMMA is less than30%.
 20. A cosmetic composition comprising a microdispersion of acrylicparticles obtained by the method according to claim
 1. 21. A cosmeticcomposition comprising a microgel according to claim
 16. 22. Thecomposition according to claim 21 wherein it contains less than 30% byweight of said microgel.
 23. A nail polish composition comprising amicrodispersion of acrylic particles obtained by the method according toclaim
 1. 24. A nail polish composition comprising a microgel accordingto claim 16.